# Converging Lens and Sun

Gold Member

## Homework Statement

A camper is trying to start a fire by focusing sunlight onto a piece of paper. The diameter of the sun is 1.39e9 m, and its mean distance from the earth is 1.50e11 m. The camper is using a converging lens whose focal length is 10.8 cm.
(a) What is the area of the sun's image on the paper?
(b) If 0.568 W of sunlight passes through the lens, what is the intensity of the sunlight at the paper?

## Homework Equations

(a) A=.25*pi*(height of sun's image)^2 and m=-di/do
(b) I=P/A

## The Attempt at a Solution

B should not be an issue if I can find A. I do not know how to find the height of the sun's image.

## Answers and Replies

Homework Helper
Gold Member
Try 1/f = 1/do + 1/di

This is the "thin lens" formula. You'll use it a lot when studying lenses. You might want to commit it to memory.

Gold Member
Try 1/f = 1/do + 1/di

This is the "thin lens" formula. You'll use it a lot when studying lenses. You might want to commit it to memory.

Thanks for the reply! I really appreciate your quick response! That is one of the main formulas that we use.

I managed to solve the problem by taking the focal length over the distance earth to sun and then multiplying that by the diameter of the sun.
(.108/ 1.5E-11) * 1.39E9= h
then taking pi/4 * h^2 =A

My problem before was that I was using 10.8 instead.